1. Field of the Invention
The present invention generally relates to a numerically controlled machine tool, such as a horizontal boring machine. More specifically, the invention relates to an apparatus for compensating position errors resulting from fluctuation of the center of gravity caused by the horizontal movement of a ram carried by a spindle head.
2. Description of the Prior Art
In a typical horizontal boring machine, a spindle head moves in the vertical direction along a guided on a column, and a ram carried by the spindle head moves in the horizontal directions. If the ram moves in the horizontal direction, there is caused a phenomenon that the spindle head is inclined by the variation of the center of gravity. In the case of a large-sized horizontal boring machine, the ram itself carries heavy weight, so that the position of the center of gravity in the spindle head when the ram does not move is greatly different from that when the ram is fed to the end. By this position fluctuation of the center of gravity, the ram can not keep the straightness during the horizontal movement, so that position errors of the spindle at the front end of the ram in vertical directions are arose. Therefore, it is required to carry out any compensation in order to insure machining precision.
As a conventional method for compensating the position error due to changes of center of gravity, there is known a method for automatically hydraulically changing a rate of lifting forces of right and left two wires for supporting the weight of a spindle head, to hold the straightness of a ram during the horizontal movement.
However, the hydraulic compensation for position fluctuation of the center of gravity is restricted by a hydraulic system which has a response lag and a bad follow-up performance, so that the actual position of the front end of the spindle is different from the corrected position thereof.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an apparatus for compensating position errors of the spindle head resulting from the position fluctuation of the center of gravity in a spindle head, which is capable of responsively and precisely keep the straightness of a movable member such as a ram during the horizontal movement even if the position of the center of gravity is changed with the feeding of the movable member.
In order to accomplish the aforementioned and other objects, there is provided a compensator for compensating a vertical deviation of the movable member resulting from the fluctuation of the center of gravity of the spindle head caused by the feed motion of the movable member, said compensator comprising a Y-axis control means for controlling the first and second Y-axis servomotors on the basis of spindlehead feed instructions provided from the numerical control unit, a Z-axis control means for controlling the Z-axis servomotors on the basis of a movable member feed instruction provided from the numerical control unit; correction data storage means for storing correction data determining the relationship of a feed distance of the movable member to a compensation value for correcting a vertical deviation of the movable member; and correction means for being introduced the movable member feed instruction provided from the numerical control unit, and for reading out the compensation value corresponding to the feed distance of the movable member from the correction data storage means, to correct the spindlehead feed instruction by adding or subtracting the compensation value to or from the spindlehead feed instruction.
According to the present invention, the first servomotor rotates the ball screw by a larger amount than that corresponding to the spindle moving command before correction, to move the spindle head upward, and the second servomotor rotates the ball screw by a smaller amount than that corresponding to the spindle moving command before correction, to move the spindle head downwards, so that the attitude of the spindle head is maintained to be horizontal as a whole. Thus, the guide face of the ram is held so as to be horizontal, so that it is possible to prevent errors from being caused at the position of the tip of the ram (main spindle).